1. Field of the Invention
This invention relates to electromagnet driving circuits, and, in particular, to a driving circuit for controlling energization of an electromagnet for use in various machines such as impact printers, wire-dot printers, relay devices and buzzers. More specifically, the present invention relates to an electromagnet driving circuit for use in an impact printer for controlling energization of a driving coil which causes a printing hammer having an armature to move to apply an impact force on a selected type.
2. Description of the Prior Art
Electromagnets are well known in the art and used in various machines such as printers, relays, vibrators and buzzers. In particular, in impact printers such as a wheel printer which uses a print wheel, often referred to as "daisy wheel", an electromagnet forms an essential part as a means for driving to move a printing hammer. Since an electromagnet consists of a core and a coil wound around the core, when it is applied to impact printers, its core part is formed as a part of a printing hammer in the form of an armature and its coil is provided stationarily, whereby the coil is intermittently energized to cause the printing hammer to advance to hit a selected type carried by a print wheel. Thus, an electromagnet driving circuit is typically a circuit for controlling energization of a coil of electromagnet. When such an electromagnet driving circuit is desired to be controlled at high accuracy, it is commonly so structured to be driven by a constant current.
FIG. 1 illustrates a typical constant current type electromagnet driving circuit which has been used conventionally. As shown, it includes a coil MAG forming a part of an electromagnet to be driven, whose one end is connected to a voltage source V and whose the other end is connected to a collector of an NPN transistor Q.sub.3, which, in turn, has its emitter connected to ground through a resistor R.sub.9 or a capacitor C.sub.1 and its base connected to a collector of a PNP transistor Q.sub.2 through a resistor R.sub.8. A protective diode D.sub.2 is connected between the ends of the coil MAG. The emitter of transistor Q.sub.3 is also connected to a non-inverting input terminal of an operational amplifier OP through a resistor R.sub.6, and this non-inverting input terminal is also connected to another voltage source V.sub.Z through a resistor R.sub.3. The op amp OP has its inverting input terminal connected to receive a reference voltage V.sub.r which is generated by a voltage divider consisting of resistors R.sub.1 and R.sub.2 which are connected between the voltage source V.sub.Z and ground in series. The op amp OP has its output connected to its non-inverting input terminal through a feed-back resistor R.sub.4 and to the base of transistor Q.sub.2 through a resistor R.sub.5. Transistor Q.sub.2 has its emitter connected to an emitter of transistor Q.sub.1 which has its collector connected to a 5 V voltage source and its base connected to an input terminal to which a driving (control) pulse DRV is applied. Diodes D.sub.1 and D.sub.3 and resistors R.sub.7 and R.sub.10 are additionally provided as connected as shown.
In FIG. 1, i.sub.M indicates a driving current which passes through the coil MAG and V.sub.a indicates an input voltage to the non-inverting input terminal of op amp OP with V.sub.0 indicating an output voltage of op amp OP.
The driving current i.sub.M passing through the coil MAG is detected as a voltage drop across the resistor R.sub.9 having a relatively small resistance value, and the voltage at the junction between the resistor R.sub.9 and the emitter of transistor Q.sub.3 is supplied to the non-inverting input terminal of the op amp OP through the resistor R.sub.6. On the other hand, the reference voltage V.sub.r determined by a ratio in resistance value between the two resistors R.sub.1 and R.sub.2 and the voltage level of the voltage source V.sub.Z is applied to the inverting input terminal of op amp OP, so that the op amp OP compares these two input voltages and controls the ON/OFF condition of transistor Q.sub.2 according to the result of such comparison. The ON/OFF condition of transistor Q.sub.2 thus controlled by an output of op amp OP is transmitted to the transistor Q.sub.3 as valid information only when the transistor Q.sub.1 is turned ON by receiving the driving pulse DRV. In this manner, the driving current i.sub.M is maintained at a predetermined level as a constant driving current.
FIG. 2 shows a relation between the driving pulse DRV and the driving current i.sub.M in the circuit of FIG. 1. In FIG. 2, I.sub.M indicates a predetermined level of a desired constant current and t indicates time.
The constant current level I.sub.M is related to the reference voltage V.sub.r with the following equation in the circuit of FIG. 1. ##EQU1## where, V.sub.r =V.sub.a.
Moreover, from the voltage waveform shown in FIG. 2, an energy W supplied to the coil MAG by the driving current i.sub.M may be expressed as follows: ##EQU2## where, V.sub.CE : collector-emitter voltage of transistor Q.sub.3
R.sub.M : internal resistance of electromagnet MAG PA1 .tau..sub.1 : time constant for current rise PA1 .tau..sub.2 : time constant for current fall due to fly back voltage.
As is obvious from the above equations (1)-(3), when the voltage source V fluctuates, the current rise time t.sub.1 shifts to t.sub.1 ' or to t.sub.1 " as shown, which, in turn, will cause the energy W to change accordingly. Stated more in detail in this respect, when the source voltage V changes, the current rising characteristic changes as indicated by the dotted lines in accordance with changes in the voltage source V. That is, when the voltage source V increases, t.sub.1 shifts to t.sub.1 '; on the other hand, when V decreases, t.sub.1 shifts to t.sub.1 ". Under the condition, if the pulse width of driving pulse DRV (t=0-t.sub.2), which determines a time period of passing current through the coil MAG, is maintained at constant, an integral value of i.sub.M from t=0 to t=t.sub.3 will vary depending upon the rising characteristics of the driving current i.sub.M. Thus, the energy W supplied to the coil MAG will differ according to the equation ( 2). As set forth above, even if the driving current i.sub.M is maintained at constant, the energy W given to the coil MAG will differ when the voltage source V connected to the coil MAG fluctuates. For this reason, if the coil MAG or its electromagnet as a whole requires to be driven by a constant energy, the driving circuit of FIG. 1 is inappropriate.
As described above, in some applications, it is rather important to drive an electromagnet or its coil with a constant energy rather than driving current in order to attain desired objectives. For example, in the case of impact printers, it is often required that a printing hammer be driven at constant energy so as to form imprints of uniform density. In such a case, the constant current type electromagnet driving circuit as described above is not sufficient. Therefore, there has been a need to develop a novel electromagnet driving circuit whose driving energy may be maintained at constant.